Miss Marple (Helen Hayes) is reunited with her old American friend Carrie Louise Serrocold (Bette Davis) from their days touring Italy as young ladies. Christian Gulbranson, Miss Marple's lawyer, persuades her to visit the estate of his step-mother, Carrie Louise. Carrie Louise's devoted husband Lewis has turned the manor house Stonygates into a halfway house for young criminals. He confides to Miss Marple that he suspects that someone is slowly poisoning Carrie Louise with arsenic. But then Christian Gulbranson is found murdered. A murderer is at large in a house full of suspects.